The following invention relates to a fluxmeter and more particularly to a fluxmeter having digital circuitry to provide a high degree of stability and accuracy.
Fluxmeters are devices which measure total magnetic flux linking a sensor coil. The fluxmeter measures total flux, which is the integral of flux density times area over the area of the coil. The output of the fluxmeter is proportional to the number of turns on the coil as well. Typically, a fluxmeter uses a coil of electrically conductive wire which is often chosen by the user for the particular purpose at hand.
For a coil circling a flux path, the Faraday Induction Law states that: EQU E=nd.phi./dt.
Therefore, the total flux (.phi.) is proportional to the integral of the voltage over time and inversely proportional to the number of turns in the coil: EQU .phi.=1/n.intg.Edt+.phi..sub.0.
In conventional fluxmeters the meter is set to zero before the measurement is made so that .phi..sub.0 =0. The output may also include a multiplier correction for the number of turns in the coil or scale changes for the output display.
Conventional fluxmeters are analog devices and typically employ an operational amplifier with a shunt capacitor to integrate the voltage over time. Two different problems are encountered with this type of device. The first is that the input impedance of the meter varies with the output scale setting. If the input impedance is low enough, and if the sensor coil has significant resistance, a correction is required which must be computed and which is different for each output scale setting. Additionally, analog integrators are highly susceptible to drift. Any small offset voltage produces a gradual change in the output, even with no actual change in flux. The offset voltage may arise as a result of electrical and magnetic noise. The drift may be so large, however, that it may be difficult or impossible to obtain accurate, reliable and repeatable readings.
A typical fluxmeter is shown in a paper by Sasaki "A Simple Precision Fluxmeter", Nuclear Instruments and Methods 76, North Holland Publishing Company (1969) pp. 100-102. The Sasaki fluxmeter states that it employs an integrating digital voltmeter, but in reality Sasaki's integrator is a conventional analog integrator with digital processing of the integrated output. This device is therefore susceptible to the type of drift discussed above. Another type of fluxmeter is shown in the U.S. Patent to Krause, U.S. Pat. No. 5,506,500. Krause appears to show the use of a digital voltmeter as a fluxmeter, however, the voltmeter is not truly a digital device, but instead contains a precision analog integrator comprising an OP amp and capacitor. It is therefor susceptible to the problem of drift as outlined above. Furthermore, Krause relies upon a stepper motor to move a sample between a coil pair. The mechanical dimensions of the device are carefully controlled as are the starting and stopping of the motor which controls the timing of a measurable event. With manually operated fluxmeters however, there is no such degree of control, and events of interest must be determined by the instrument itself which must discriminate frequently between the measurement of magnetic flux and readings caused by noise, such as noise from nearby AC-lines, fluorescent lighting or machinery.